This invention relates generally to methods for computed tomography (CT) imaging and more particularly to methods and apparatus for calibrating CT imaging apparatus detectors to produce images having reduced artifacts.
X-ray CT measures x-ray attenuation through an object with a plurality of detector elements at various angular positions with respect to the object. With appropriate data processing, a cross-sectional image revealing inner structures of the object can be reconstructed. Reconstructed images represent a property of an object in response to x-rays (or other radiation), namely, the x-ray (or other radiation) linear attenuation. To reconstruct artifact free images, calibration of an CT imaging system and an appropriate correction must to be applied in the reconstruction process. For example, calibration and correction related detector response to incident x-ray spectrum must be performed. X-ray spectra often have wide energy bandwidths, typically from a few tens of keV to more than a hundred keV. In third generation CT configurations an X-ray tube and detectors rotate together around an object. This configuration makes image quality sensitive to spectral characteristics of detector elements. This sensitivity tends to be greatest for detector elements near iso-center, the axis around which the CT gantry rotates. Thus, effort has to be expended to produce and select detector elements with nearly identical spectral characteristics.
After detector elements are arranged in a detector array, a spectral calibration can be performed to determine residual spectral response differences among detector elements and to ensure that reconstructed images of water phantoms are uniform and free of artifacts. Inaccurate determination of spectral response differences between detector elements results in rings and bands in images around the iso-center. In at least one known method for spectral calibration, uniform phantoms made of water-like material are used for calibration. By scanning these phantoms, the detector response to an x-ray spectrum attenuated through a given length of water can be determined. However, multiple phantoms of various sizes are often required to cover the attenuation range needed for some types of imaging, including medical imaging, leading to lengthy calibration times. Spectral calibration is often performed every few months. Because of the complexity of the calibration procedure, well-trained technicians must usually be employed to perform these calibrations.